1. Field of the Invention
The present invention relates to the thermal treatment of glass sheets. More specifically, it relates to the heat strengthening of glass sheets used in large window openings where it is important that the glass have suitable break patterns that prevent the glass from forming islands within the supported marginal periphery when the window fractures. These islands fall from the main body of the glass when the glass is fractured and are dangerous to people passing by a building containing such windows, especially when the windows are installed in an upper floor of a skyscraper.
When glass sheets are either tempered or heat strengthened to a partial temper by conventional techniques of heating the glass to above its strain point to attain a temperature sufficient for tempering followed by cooling the glass at a controlled rate of cooling, or when the glass sheets are heat treated by heating the glass sheets to a temperature sufficient for tempering followed by natural cooling, typical break patterns sometimes occur having particles in the form of islands that separate from the main body of the glass sheet when the sheet is fractured. The size of the particles or islands that are formed is dependent upon the degree of temper developed in the glass. Generally, large particles are associated with glass sheets that are heat strengthened to a relatively low temper and the size of the particles in general become smaller as the temper in the glass becomes greater.
It is well known that when glass sheets are subjected to a thermal treatment consisting of heating followed by rapid cooling, that the glass sheets develop a temper. The degree of the temper depends upon the elevated temperature to which the glass sheets are heated and the rate of cooling from the elevated temperature to below the strain point of the glass. Tempered glass sheets develop a compression stress zone in their edge and surface portions that surround an interior portion stressed in tension. Since glass is extremely strong in compression and extremely weak in tension, as long as any mechanical or other force applied to the tempered glass is insufficient to overcome the compressive stress at its surface or edge portion, tempered glass resists breakage. However, when tempered glass sheets fracture, they form relatively small particles that separate readily from a frame in which the window is installed in a building. Fragments dropping onto a pavement below the window are dangerous to passersby.
Uncontrolled tempering of glass sometimes causes glass warpage which results in mechanical stress during and after installation. Inducing a partial temper by controlling heating and/or cooling has been used to develop windows that are warped a minimum amount so that they can be installed without excessive stresses within a window frame. However, under some circumstances, certain heat strengthened glass sheets provided with a limited degree of temper have fractured in the past, and the resulting break pattern provided at least one large piece in the form of an island that fell out of the remainder of the window installed within an installation frame.
When fabricating windows for skyscrapers, considerable breakage has occurred. Glass sheets have been heat strengthened in an attempt to both minimize warpage and provide sufficient temper to enable the window to resist fracture under most conditions encountered during handling and usage. However, when heat-strengthened windows break due to stresses resulting from mechanical or heat forces applied locally to portions thereof, such windows develop a break pattern. Under certain conditions, the break pattern provides a line of breakage that is almost continuous to form one or more discrete areas inward from the frame that supports the window by engaging its marginal portion. Large pieces of glass within said frame sometimes separate from the window and fall onto the sidewalk below.
Attempts to develop a better break pattern that does not result in a break line enclosing a discrete area spaced inward from the installation frame have been successful prior to the present invention only with heat strengthened glass sheets that have a much lower stress pattern than tempered glass. A suggested method of producing a lower stress pattern involves a slower rate of forced cooling, even to the point of letting the glass cool naturally or at a restricted rate of cooling. It has been found that under some conditions not yet identified, such naturally cooled glass also develops a break pattern that includes a break line surrounding a discrete area or island of rather large dimensions that tends to break away from the remainder of the glass sheet. Reducing the rate of applying cold air to the opposite surfaces of the heated glass sheets reduces the stress in the glass but, because the residual compression stress reduction is insufficient, fails to avoid more frequent breakage associated with lesser residual compression stresses that were characteristic of glass sheets having an acceptable break pattern.
Prior to the present invention, a need existed for a window which had a higher residual edge compression stress coupled with a more uniform residual surface compression stress so that the window would be less likely to break and, if broken, would be retained within an installation frame. A need also existed for an improved method for making a window having a break pattern that insured the likelihood of such an event that would avoid the dropping of large pieces from windows onto pedestrians below a building in which the window is installed. Generally, glass sheets were rendered less susceptible to thermal breakage and to edge damage during handling and installation by imparting a high residual compression edge stress to the glass. However, glass sheets thermally treated to have a high residual compression edge stress also had a steep gradient of residual surface compression stress throughout the extent of the glass sheets. The prior art did not fully appreciate how to avoid the steep gradient of residual surface compression stress while developing a higher residual surface compression stress in the glass.
A method was developed to cool glass sheets from the elevated temperature associated with tempering in such a manner that as the glass sheets passed through the cooling station of heat strengthening apparatus, a more uniform residual surface compression stress pattern resulted in the interior portion of the glass sheet surrounded by its edge. However, the techniques developed prior to the present invention failed to develop surface compression stresses over the major surface of the glass that meets the minimum standards of surface compression stress established in Federal Specification DD-G-1403B.
2. Description of the Prior Art
U.S. Pat. No. 2,093,040 to Eckert teaches a two step method of tempering glass sheets in which the glass sheets are initially chilled as rapidly as possible to a temperature which lies at or near or somewhat below the annealing temperature of the glass, i.e., that temperature below which temporary stresses are mainly developed. According to this patent, further cooling is accomplished at a slower rate, but one that is still more rapid than a natural cooling rate in an open air environment.
U.S. Pat. No. 2,188,401 to Crowley discloses apparatus for tempering glass sheets in which a plurality of rotatable shutters are interposed between upper and lower sets of nozzles to insure that the entire length of a glass sheet is cooled from the same instant at the beginning of a cooling step as all other portions of the sheet so as to minimize the danger of warpage or breakage of the sheet during its fabrication.
U.S. Pat. No. 3,251,670 to Acloque interposes a disc or a donut-shaped member between tempering nozzles and a portion of the glass sheet to be provided with less temper than the remainder of the sheet in a technique for differentially tempering glass sheets. Other patents showing deflectors or angle bars interposed between a source of pressurized air and the opposite major surfaces of a glass sheet to be differentially tempered inclusde U.S. Pat. Nos. 3,363,936 and 3,396,001 to Baker and U.S. Pat. No. 3,364,006 to Newell et al.
Furthermore, U.S. Pat. No. 3,304,166 to Bolland discloses the use of screens for reducing the rate of flow from high pressure air blasting members against a localized portion of a glass sheet to be tempered to a lower stress than the remainder of the sheet during differential tempering.
U.S. Pat. No. 3,847,580 to Misson discloses a two step cooling method for tempering glass sheets while supported on a gaseous hearth. During the first step, the glass is supported and chilled rapidly until both its center plane temperature and its surface temperature is reduced below the strain point of the glass. During the second step, the glass is supported and cooled by relatively lower volumetric flows of cooling gas per unit of surface support area to maintain the temper initially imparted during the first step. The total power consumption for this two stage tempering process is less than that required for conventional tempering in which the high rate of cooling is maintained throughout cooling.
U.S. Pat. No. 4,236,909 to Thomas, Frank and Claassen obtains an improvement in the break patterns developed by a technique in which the glass sheets, after heating to an elevated temperature sufficient for tempering, are initially force cooled at a rate sufficient to develop a temperature gradient from its major surfaces to the center of its thickness that is steeper than the temperature gradient produced by natural cooling, and before the glass sheet cools to the strain point at the center of its thickness, discontinuing the force cooling and retarding the rate of glass sheet cooling at the major surfaces to a rate less than the rate of cooling the edge surfaces of the glass sheet by supporting the glass sheet immediately after the force cooling with its major surface facing continuous walls closely spaced relative to the major surfaces of the glass sheet while the edge surfaces face the space between the walls until the sheet develops a more uniform surface compression stress pattern over the entire extent of the glass sheet within an edge portion that is relatively highly stressed in compression. This technique was used in an attempt to obtain higher residual surface compression stresses. The break pattern that developed at higher surface compression stresses was insufficient to provide the safety factor that was obtained at the lower residual surface compression stresses obtained with this technique for glass sheets heat strengthened to a lower degree of temper.